In the case of interchangeable cutting inserts, generally consisting of tungsten carbide, it is important that on their side turned away from the effective cutting edge, they are supported on the holder over the widest possible surface. For this reason, on the holder or on an insert carrier inserted in the holder, there are provided at least two bearing surfaces extending at an angle to each other. In the hexagonal cutting inserts, as frequently employed, which have an angle of somewhat less than 90.degree. at every other corner, it would be desirable for ensuring a non-rotational and dimensionally accurate seating if these cutting inserts were supported on three bearing faces. The maximum importance should be given to a satisfactory support in order, on the one hand, to fix the cutting insert in a dimensionally exact manner with respect to the holder, and on the other hand, to ensure transmission of the forces set up in machining from the cutting insert to the holder, without at the same time damaging the unused cutting edges. The production of these bearing faces which must not only extend at an angle to each other corresponding to the outer contour of the cutting insert, but also exactly at an angle determined by the shape of the cutting insert with respect to the seating surface, requires expensive special machine tools. In the case of complicated tools, the production of the bearing faces encounters difficulties if there is little room and there are no orientation points for dimensionally correct positioning of the cutting insert. Frequently, the user of the turning tools would also like to make for himself the actual tool holders, such as turning tool holders, boring tool holders, drill heads and other special tools and fix on these holders merely the cutting inserts made by another firm. This, however, has hitherto scarcely been possible owing to the bearing faces being difficult to make.
It is true that a turning tool of the above-mentioned kind is known, in which the cutting insert is secured not directly to the holder but to an insert carrier insertable in the holder. The insert carrier has the accurately machined bearing faces for the cutter insert. Its external form, however, is of such a shape that the necessary recess for the insert is simpler to make on the holder. A known insert carrier has a substantially U-shaped contour. Two mutually parallel lateral faces are connected together by a semicylindrical supporting surface. On its underside the insert carrier has a plane base face. By means of an end-milling cutter a recess for receiving the said insert carrier can be made in the holder with comparative ease. This known insert carrier, however, cannot be used in many cases since its external dimensions are too large compared with the cutting insert. This is to be attributed chiefly to the fact that the cutting insert is held on the insert carrier by means of a clamping claw, which is supported by its front end on the cutting insert and by its rear end on the insert carrier. For accommodating this clamping claw and a clamping screw holding the clamping claw, the insert carrier must be made correspondingly large. Owing to its external dimensions, the known insert carrier also requires a comparatively large recess, resulting in considerable weakening of the holder. For this reason also, this known insert carrier often cannot be used.
Another turning tool is also known (from U.S. Pat. No. 2,630,725), in which the insert carrier in plan has a substantially rectangular shape. The cutting insert is fixed at one corner of this rectangle by means of a clamping screw inserted in a central bore of the cutting insert. In the region of the diagonally opposite corner, the insert carrier has a screwthread for screwing in a holding screw, by means of which it is fixed to the holder. This known insert carrier also has the disadvantage that it requires comparatively much room, added to which is the fact that the rectangular recess necessary for its accommodation in the holder is difficult to make.
It is the object of the invention to provide a turning tool having a replaceable polygonal cutting insert of the type referred to in the foregoing, in which the insert carrier has the smallest possible dimension and the recess necessary for it in the holder is simple to make.
This is achieved according to the invention in that the cutting insert, in the manner known per se, has a central bore for the clamping screw, the axis of the semicylindrical supporting surface coincides completely or approximately with the axis of the central bore and a screwthread or a bore for the holding screw is provided in the insert carrier below the cutting insert. Preferably, the radius of the supporting surface is only slightly larger than the distance of the cutting points from the axis of the central bore.
In the turning tool according to the invention, the insert carrier has particularly small dimensions. In the cutting plate plane, it is scarcely larger than the cutting insert itself. Below the cutting insert, it has a height, corresponding approximately to the height of the cutting insert. Since however, in most turning tools, shim plates are provided in any case between the holder and cutting insert, the space requirement of the cutting insert and the insert carrier in the case of the turning tool according to the invention is only slightly greater than the space requirement of a cutting insert with a shim plate. Owing to its small space requirement, the insert carrier can be used practically everywhere, where hitherto the cutting insert has been mounted directly in the holder. The simple external contour of the insert carrier with an approximately semicylindrical supporting surface and plane base face requires in the holder a recess which can be made very simply by means of an end miller. For this reason, the users of turning tools can make these themselves if necessary. They merely order from another manufacturer the cutting plates and insert carriers which can then be made by means of suitable special machines with high accuracy, and since they have always the same form, independently of the particular turning tool, they can be made in large numbers.
A particularly advantageous embodiment of the turning tool according to the invention is that the insert carrier has a fixing bore extending perpendicularly to its base face for a fixing pin engaging the fixing bore and a corresponding bore of the holding recess. By means of this fixing pin, the insert carrier is fixed exactly relative to the holder and rotation or displacement of the insert carrier relative to the holder is avoided.
Further advantageous embodiments are characterised in the sub-claims.